International Ocean Drilling Programme (IODP³) D/V Chikyu Completes Expedition 503: Hadal Trench Tsunamigenic Slip History

The Deep-Sea Scientific Drilling Vessel Chikyu^*1, operated by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC: Hiroyuki Yamato, President), has completed the International Ocean Drilling Programme (IODP³)^*2 Expedition 503: Hadal Trench Tsunamigenic Slip History, on 12 December (Fri) 2025, having achieved its objectives.

1. Operation Summary

This expedition started on 24 November 2025, where drilling using the deep-sea drilling vessel Chikyu (previously reported on October 20, 2025), to drill a trench basin in the central Japan Trench to recover the whole trench-fill sequence for dating and establishing event-stratigraphy for paleoseismologic interpretations and further investigations of earthquake related element cycles in hadal trench environment. (Fig. 1, Fig. 2).

In this expedition, core sampling using the Hydraulic Piston Coring System (HPCS) was conducted in five boreholes: C0028A, B, C, D, and E. As drilling took place in waters exceeding 7,600 meters depth, where the vessel's Underwater TV system (UWTV) could not confirm the seafloor surface, drilling commenced only after the HPCS had accurately verified the seafloor depth. The drilling at a depth of 7,608.5 meters in borehole C0028E represents the deepest drilling operation using drill pipes in scientific drilling history, surpassing the previous record of 7,034 meters set by the US scientific drilling vessel Glomar Challenger in the Mariana Trench in 1978. Subsequently, Chikyu entered Shimizu Port (Okitsu Second Pier), Shizuoka Prefecture, on 10 December, concluding this expedition on 12 December.

2. Overview of Results and Future Plans

During this Expedition, core samples were collected from the entire stratigraphic sequence of trench-filling sediments deposited in the central Japan Trench. Using these core samples, the following objectives were pursued: to determine the age of event deposits generated by shallow slip at the plate boundary and by changes associated with large earthquakes using various methods, thereby clarifying their occurrence intervals and past history; The research aims to analyze the pore water contained within the sediments to determine its origin, assess the contribution of the fluid to earthquakes or the impact of earthquakes on the fluid, and investigate the effects of sediments transported into the trench by earthquakes on the subsurface biosphere and their relationship with material cycles.

3. Expedition website

A dedicated website for the expedition was launched. This website provides an overview of the expedition, introduces the participating researchers, and has been updated throughout the expedition.
https://www.jamstec.go.jp/chikyu/e/exp503/

Fig.1　Survey Area

Fig.2　Drilling Plan
At drill Site C0028 (Proposed drill Site JTC-01A), five boreholes were drilled using the HPCS and samples collected as follows:
C0028A：Water depth 7,608 m / 9.5 m below seabed
C0028B：Water depth 7,608 m / 178 m below seabed
C0028C：Water depth 7,607 m / 157.5 m below seabed
C0028D：Water depth 7,607 m / 86.5 m below seabed
C0028E：Water depth 7,608.5 m / 7.0 m below seabed

Photo 1. (27 November) The process of extracting pore water from core samples

Photo 2. (4 December) Core samples for which Scientists on board have submitted requests for sampling